It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.
Geopolymers are synthetic materials produced by polymerization of a polymerizable material under alkaline conditions. These materials can provide comparable performance to traditional cements or concretes, and can also exhibit properties including high compressive strength, low shrinkage, fast or slow setting, acid resistance, fire resistance and low thermal conductivity.
Geopolymers also have an added advantage in that they can be produced from waste materials such as fly ash. Fly ash is an aluminosilicate fine particle residue collected from flue gas after the combustion of coal in a coal-fired power station. Globally more than 500 million tonnes of fly ash is produced annually, and a substantial amount is disposed of in landfills and/or lagoons. Fly ashes may be classified under ASTM C-618 into Classes F, C and N.
However, when for example fly ashes are used to make geopolymers, there can be a wide variation in the mechanical properties of the geopolymers formed. For example, Diaz-Loya et al (2011) studied the mechanical properties of geopolymer concretes made from different fly ashes collected from power stations across the United States. The result showed that the geopolymers made from 11 Class F fly ashes had an average compressive strength of approximately 36 MPa, while the geopolymers made from the remaining 11 Class C fly ashes had an average compressive strength of approximately 50 MPa. One geopolymer from a Class C fly ash only achieved a compressive strength of 2.73 MPa while some geopolymers from Class F fly ashes exhibited compressive strengths around 12 MPa. Three fly ashes were excluded as they were neither Class F or Class C according to their SO3 content and particle size distribution.
Other researchers have also found similar variations when Australian fly ashes from different sources were used to make geopolymers. When the same activation and curing conditions were used, some fly ashes generated geopolymers with high compressive strength while other fly ashes were poorly activated and were unsuccessful in forming geopolymers (Keyte 2008; Provis et al. 2009).
A reason for the large variation in the geopolymer products is the highly heterogeneous nature of fly ash. It is reported that there are more than 13 identifiable phases in a fly ash, including C3A-rich glass and low-silica glass (which are the main phases in high-calcium fly ash) and mullite-rich glass and Class F glass (which are the main phases in low-calcium fly ash) (Bumrongjaroen, et al. 2007). Furthermore, the mineral in coal powder varies from one power plant to another, even from time to time at the same plant, and the temperature inside the boiler may vary from 800 to 1400° C. Consequently, the collected fly ash may vary significantly in particle size, shape and composition.
Therefore, it is difficult to predict what properties a geopolymer will have, based on the starting materials used in the geopolymerization reaction.
One method to quantify the reactivity of fly ashes for geopolymer synthesis is described in US2011052921, which provides a procedure to quantify the reactivity of fly ashes in alkaline solutions as a function of pH and temperature. In this document, the fly ashes were characterized by their reactivity, measured in terms of the reaction progress which is the relative mass of reacted glass in alkali hydroxide solution as a function of time. However, this procedure requires reactions to be performed on the fly ash samples over periods of up to 14 days, and at temperatures of up to 75° C., before filtering, drying, washing, filtering and then drying the sample. This procedure can be time-consuming, and the measured reactivity may be affected by many variables including the molarity and kind of alkali in the alkali hydroxide solution, the temperature and the water-to-solid-ratio.
Other methods have focused on manipulating the activation conditions to achieve the desired geopolymer properties, such as the maximum mechanical properties. This may include manipulating the activator, additives and/or the curing temperature (Bakharev 2005; Criado, Palomo, and Fernández-Jimenez 2005; Dombrowski and Buchwald 2007). However, these conditions may require manipulation for each fly ash used.
Consequently, in one aspect an object of the present invention is to provide a method of assessing the reactivity of a polymerizable material in forming a geopolymer, based on the properties of the polymerizable material alone. In further aspects, an object of the present invention is to overcome one or more of the difficulties discussed above or to provide the consumer with a useful or commercial choice.